1. Field of the Invention
The present invention relates to ultrasonic imaging apparatus. More specifically, this invention relates to a temporal filter used for maintaining the pulsatility of displayed blood flow information while increasing the signal-to-noise ratio for low velocities of the blood flow.
2. Background of Related Art
Images of living organisms typically utilize methods that pass various types of radiation through the body of the subject and measure the output with a suitable detector. For instance, x-ray images are generated by producing x-rays external to the body, passing the x-radiation through the body and observing shadows produced on x-ray sensitive film. Ultrasonic images, in contrast, are formed by producing ultrasonic waves using a transducer, passing those waves through the subject's body, and measuring the properties of the scattered echoes from reflections inside the body using a receptor. This is done by applying the well-known Doppler effect, by measuring the phase shifts of the reflected waves from the waves passed through the subject's body. An ultrasonic imaging apparatus may be distinguished from other medical imaging apparatus in that they allow the display of soft tissues within the body which show various structural details such as organs and blood flow.
The basic principle used in applying the Doppler effect for imaging in a pulsed Doppler ultrasound imaging apparatus is described as follows. When blood flow within a living subject is subjected to ultrasonic waves, corpuscles are caused to vibrate slightly while moving and reflect those ultrasonic waves. Because of the corpuscle velocity, the frequency of the reflected waves changes from that of the transmitted waves due to the Doppler effect. The frequency shift may be detected and the amount of the shift may be used to display velocity information of blood flow on a video screen for imaging the living subject. Because the amount of phase shift of the reflected waves from the transmitted waves is proportional to the blood flow velocity, the amount and speed of the blood flow may be determined. Noise and other signals (clutter) which have Doppler shift but don't represent blood movement in the subject are filtered out, so that the image produced only represents blood flow. In color Doppler imaging the frequency information is then used as blood flow information for forming a two-dimensional image or profile of the blood flow velocity.
One such apparatus used in displaying information obtained from ultrasonic reference pulses transmitted into a living subject is shown in FIG. 1 as imaging system 100. Imaging system 100 generally comprises a probe 101 which is coupled via line 110 to transmitter/receiver circuitry 102. Transmitter/receiver circuitry 102 is designed so that the elements in probe 10 1 (or the single element in a motorized probe) will be fired at specific time intervals to simulate an elliptical surface and focus on a particular point in the body. Reflected signals are detected using a receiver in probe 101 at a second time interval. Transmitter/receiver circuitry 102 is coupled to a control unit 109 via bus 120. Control unit 109 controls circuitry in the imaging system via bus 120 such as timing of the reference pulse and operation of the receiver in probe 101. Control unit 109 is further coupled to a keyboard 125 and a mouse, trackball or other device 126 for movement and control of information shown on video display 130.
Once a pulse is received by the receiver circuitry within transmitter/receiver 102, such information is transmitted by line 111 to RF (radio frequency) processor 103 for further processing. RF processor 103 processes the RF information to produce an envelope signal and in-phase (I) and quadrature (Q) Doppler signals. This information is further transmitted via line 114 to a scan converter 105 and to a Doppler processor 106 via lines 114 and 113 for generation of black and white ultrasound information on video display 130. Information generated by Doppler processor 106 via I and Q signals output from RF processor 103 are transmitted via line 115 to scan converter 105. Scan converter 105 then integrates information received from RF processor 103 and Doppler processor 106 and transmits scan line information to video processor 127 via line 116. In addition to information passed to scan converter 105 and Doppler processor 106, RF processor 103 transmits I and Q signals via line 112 to color flow processor 104. Color flow processor 104 is also controlled by control unit 109 via bus 120. Color flow processor 104 is used for detecting Doppler shift and blood flow information in living tissue, and thus transmits such information via line 117 to a color scan converter 108. Such color information is used to graphically represent moving blood flow in the living organism on video display 130. Color scan converter 108 is used to interpolate scan line information obtained from color flow processor 104, and transmit that information on line 118 and thus to video processor 127 for representation of blood flow in the human body. Video processor 127 then utilizes information obtained from scan converter 105 for display of black and white ultrasound information and color information obtained from color scan converter 108 to generate a color image showing blood flow in color overlaid on a black and white image showing stationary tissue suitable for output on a video display such as 130 via line 119. Such information may be transmitted in National Television Standards Committee (NTSC) format and thus be stored on video tape for later clinical examination by attending medical personnel.
One inherent problem of displaying blood flow within an organism is that when there is little phase shift from the pulse repetition frequency (PRF), (thus indicating stationary or very slow moving blood flow), there tends to be inaccurate and/or confusing representations of the blood flow. As measurements show stationary or slow moving blood flow, the actual flow may be shown in such a way as to confuse the clinician. This is graphically represented with reference to FIG. 2. As is shown in the velocity versus time graph 200 on FIG. 2, a particular blood flow may be at or near the base line 203 of the graph thus indicating very slow moving or stationary blood flow movement. Blood flow which is either stationary or moving very slowly in a particular direction may result in measurement inaccuracies by the ultrasonic imaging apparatus thereby causing blood flow to appear to be moving when it is stationary, or moving in an opposite direction than it is actually moving. This causes artifacts or a variety of colors on a display such as 130 shown in FIG. 1. In addition, because stationary or very slow moving blood flow may be moving very slightly, small areas in the flow appear to be flowing towards the transducer or away from the transducer. In a two-color display showing blood movement in one direction as red (red-shifted) or blue (blue shifted), this may be very confusing to an operator or attending clinical personnel. In other words, the display in the stationary or slow moving area may appear as a series of blue and red pixels interspersed. This may mask actual problems that the subject is experiencing or show problems where none exist.
One prior art method to limit the amount of distortion which occurs on display 130 due to little or no blood movement, is the use of a temporal filter. Once such temporal filter which may be present in a video processor such as 127 shown in FIG. 1 is shown as 300 in FIG. 3. In this prior art system, input and output lines comprise 8 bits of color information for 256 possible color levels which may be displayed for each pixel on a screen such as 130 shown in FIG. 1. 300 has two signal paths 310 and 320 holding the new color data for a pixel to be displayed on video display 130, and old pixel data contained on line 320 which holds a previous time period's pixel information. These two signal lines are input to two multipliers 301 and 302 which generate the product of input data obtained from lines 310 and 320, and data contained on lines 311 and 321. Multiplier 301, in addition to receiving new data 310, receives an alpha value (.alpha.) over line 311. In this example, .alpha. varies between 0 and 1. Multiplier 301 generates the product of .alpha. received over line 311 and the new data received over line 310. Multiplier 301 generates the product of the multiplication on line 312. Multiplier 302 accepts old pixel data over line 320 and a value received over line 321 which contains 1-.alpha.. The old data on line 320 is obtained from the previous output of circuit 300 on line 330 which is fed back through delay circuit 305. Delay circuit 305 delays the original information from a previous frame so that it can arrive at multiplier 302 at the same time as the new frame data over line 310 arrives at multiplier 301. A product is generated by multiplier 302 over lines 322. Lines 312 and 322 are input to an 8-bit adder circuit 303. Then, the resulting 8-bit sum generated by adder 303 is output over lines 330, and ready for display on video display 130 as shown in FIG. 1. This information may also be input to a delay circuit 305 in order to generate the old pixel value for the new frame at lines 320. In short, the circuit shown as 300 in FIG. 3 is defined by the following equation: EQU output pixel=.alpha..multidot.data.sub.new +(1-.alpha.)data.sub.old
The output value is therefore a temporal weighting of an old pixel's value and a new pixel's value. This provides a smooth transition between the old data residing at that position and the new data. A temporal filter having a fixed value of .alpha. as shown in FIG. 3 also has some undesired effects.
FIG. 4 shows the results of applying a temporal filter with a fixed value of .alpha. applied across the color signal range as was shown in FIG. 2. As is shown in waveform 400 in FIG. 4, although region 401 (corresponding with region 201 in FIG. 2) has been substantially smoothed, the peaks such as 202 in FIG. 2 have also been smoothed as shown by 402. In other words, because an average between the previous signal and the current signal has been perforated, large variances in the signal have been eliminated. Although the distortions which previously occurred in region 201 have been eliminated, serious distortions have been caused in area 402 of curve 400 of large variances in the original signal such as in region 202. In other words, the pulsatility in a region such as 202 in FIG. 2 (which may be caused in an artery due to the pumping action of the heart) has been comprised. Again, this major distortion may add to confusion and/or misdiagnosis by clinical personnel because high velocity data at a discrete interval in time is averaged heavily and does not reach its actual maximum value on the display. Another problem occurs due to the averaging of a previous signal and a current signal, in that unacceptably long decay times for colors on display 130 may occur. For instance, very long delay times may result in data being displayed even after the probe is no longer receiving information. In summary, applying a fixed value of .alpha. to perform temporal filtering of blood flow imagery has several undesirable effects.